Alkaline secondary batteries such as nickel-cadmium batteries have conventionally been used as driving power sources or backup batteries for electronic apparatuses. As the performance of such electronic apparatuses are improved, nickel-hydrogen batteries are increasingly used which have a higher capacity and a longer lifetime than the nickel-cadmium batteries.
The performance of a nickel-hydrogen battery depends on the activity of a hydrogen-absorbing alloy serving as a negative-electrode active substance. Therefore, a storage battery of this type employs fine powder of the hydrogen-absorbing alloy. The fine powder of the hydrogen-absorbing alloy has a large surface area for an electrochemical reaction and can be filled in an electrode substrate at a high density, thereby increasing battery capacity.
However, the hydrogen-absorbing alloy which is an extremely active substance is susceptible to oxidation during pulverization thereof to form an oxide film on a surface thereof. The oxide film reduces the electrical conductivity of the alloy and inhibits initial activation of the alloy. In view of the foregoing, there have been proposed various treatment methods for recovering the activity of the alloy subjected to the oxidation during the pulverization process, as follows:
(1) A method disclosed in Japanese Unexamined Patent Publication No. 3-98259 (1991) in which a powdery hydrogen-absorbing alloy is treated with hot water at a temperature of higher than 60.degree. C.;
(2) A method disclosed in Japanese Unexamined Patent Publication No. 4-179055 (1992) in which a powdery hydrogen-absorbing alloy is soaked in an acid solution and then rinsed with water;
(3) A method disclosed in Japanese Unexamined Patent Publication No. 4-98760 (1992) in which a powdery hydrogen-absorbing alloy is soaked in an acid solution and then in an alkaline aqueous solution, and rinsed with water; and
(4) A method disclosed in Japanese Unexamined Patent Publication No. 3-49154 (1991) in which a powdery hydrogen-absorbing alloy is mixed with a binder to form a slurry, and a phosphate or silicate is added to the slurry to suppress the oxidation of the hydrogen-absorbing alloy when the alloy is pulverized or stored.
However, the aforesaid methods suffer from the following problems:
(1) Problem of the art disclosed in Japanese Unexamined Patent Publication No. 3-98259
This art requires a considerable time for the treatment, and cannot provide a satisfactory effect for the removal of the oxide film or the like;
(2) Problem of the art disclosed in Japanese Unexamined Patent Publication No. 4-179055
In this art, an oxide layer is removed in a short time by soaking the powdery hydrogen-absorbing alloy in the acid solution. The oxide film and the like can be more effectively removed and the treatment is simpler than the aforesaid method (1). Further, where the powdery alloy is sufficiently soaked in an acid solution of a higher concentration, the oxide film and the like formed on the surface of the alloy can be sufficiently removed and, therefore, use efficiency of the surface of the alloy can be improved.
However, the use of the higher-concentration acid solution causes a hydroxide layer to be newly formed on the surface of the alloy during the acid treatment (the reason will be described later). The newly formed hydroxide layer results in an increase in the internal pressure of a battery at initial charging and deterioration of the high-rate discharging characteristic thereof. In addition, the yield of the treated alloy is reduced.
On the other hand, if the concentration of the acid solution is reduced in consideration of the aforesaid problem, a solid hydroxide layer on the surface of the powdery hydrogen-absorbing alloy cannot be sufficiently removed. Thus, a satisfactory effect of the acid treatment cannot be obtained.
(3) Problem of the art disclosed in Japanese Unexamined Patent Publication No. 4-98760
In this art, an oxide layer on the surface of the alloy is removed by preliminarily soaking the alloy in the acid solution, and then the surface of the powdery hydrogen-absorbing alloy is covered with a hydroxide layer having many grain boundaries to ensure a satisfactory discharge capacity in an initial charge-discharge process. However, since the alloy surface is covered with the hydroxide layer according to this art, the internal pressure of a battery at initial charging is increased, and a satisfactory high-rate discharging characteristic cannot be ensured.
(4) Problem of the art disclosed in Japanese Unexamined Patent Publication No. 3-49154
This art, in which an additive such as a phosphate is simply blended with the alloy, is effective in that the oxidation of the alloy can be suppressed during production of an electrode. However, the activity of the alloy cannot be sufficiently increased.